This invention relates to fuser apparatus for heat and pressure fusing toner images to sheets of support material such as plain paper. More particularly, the invention relates to apparatus and a method for reducing sheet curl induced into image substrates by roll fusers of the type employed in xerographic or similar machines.
In a typical electrophotographic printing or copying process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member.
After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an abhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. The pressure roll contacts the heated fusing roll to form the aforementioned nip through which the image carrying substrates pass. The pressure roll may also be coated with an abhesive layer. Alternately, these rolls may be provided with an abhesive outer layer comprising silicone rubber.
One problem encountered in electrostatic printing or xerography in office copiers and printers in which the image is fused on the paper under heat and pressure is that the process tends to cause a curl to be formed in the paper. Curled paper is difficult to handle in a receiver for the sheets. Ideally the sets should be neat with edges aligned and easy to handle by an operator or in finishing apparatus in which the set may be stitched or stapled.
The curl problems with sheets of paper stem from the treatment of the sheets in the processor. A sheet of normal paper is taken from a supply cassette or feeder, in an environment in which the paper is subject to humidity and absorbs a certain amount of moisture. The sheet is processed past a photoconductor where powder or liquid ink is applied to produce an image, and then the sheet is passed through a fuser and subjected to heat and pressure between rollers, with the result that curl is induced into normally flat sheets. Curled sheets received in a collator or sorter in bins cause failure of the device due to jamming or mis-sorting of sheets and the bin capacity of the receiver is reduced. In addition, curled sheets do not tend to form neat stacks or sets.
The gravity of the aforementioned problem varies depending on a number of factors. Large variations in Toner Mass Area (TMA) cause more or less curl. More importantly, as the TMA varies from low to high or vice-versa, the direction of curl changes. Low TMA prints typically produce curl which is directed away from the image while high TMA prints typically produce curl which is directed toward the image.
The degree of curl also is a function of paper weight. Light weight papers generally need more curl correction than the thicker heavy papers. Additionally, some papers are known to be heavy curlers.
Following is a discussion of prior art and other disclosures, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 4,571,054 granted to Edward F. Bowler, Jr. on Feb. 18, 1986 relates to the art of xerography and analogous graphic arts and more particularly to a copy sheet decurling mechanism employed in conjunction with a heat and pressure fuser for reducing or eliminating the curl induced into the copy sheets by the pressure roll of the fuser. To this end, means are provided for reverse (i.e. direction opposite to bending caused by the pressure roll) bending of the copy sheets while they are in a plastic state, that is while the sheets are still at an elevated temperature.
U.S. Pat. No. 5,237,381 discloses a sheet discharging apparatus having a curl generating device wherein a curl is generated in a sheet while the sheet is passing through the curl generating device, and a first guide disposed downstream of the curl generating device and switchingly movable between a first position where the sheet is directed toward a curved sheet path and a second position where the sheet is directed toward a direction the same as a sheet feeding direction from the curl generating device. The apparatus further has a sheet discharge guide disposed between the curl generating device and the first guide and switchingly movable in synchronism with the switching movement of the first guide to change its posture, between a position where the sheet discharged from the curl generating device is curled reversely and a position where the sheet discharged from the curl generating device is not further curled.
U.S. Pat. No. 5,066,984 discloses a device disposed in the path of paper sheets leaving a printing unit or processor such as an office copier or non-impact printer and has an arcuate concave guide and a roll spaced from the guide to form a sheet path which is curved or arched oppositely to the direction in which the sheet is curled in the processor. The space between the guide and the roller is greater than the thickness of the paper and the paper is bent in the direction opposite to its curl as it passes through the arched space, while the beam strength of the paper and the change in direction of the paper maintain adequate drive friction on the sheet. A selector isolates the de-curler when it is not needed.
U.S. Pat. No. 5,066,984 discloses a sheet stripping and guide assembly, suitable for use at the exit of an electrostatographic fusing station has lower stripper fingers that are inserted and molded integrally into a frame having upper sheet guide members.